Separation and Uses of Lignin
In the aqueous-phase biomass hydrolysis route of cellulosic ethanol production large volumes of lignin is produced as a byproduct. The amount of lignin formed depends on the type of biomass used and typical lignin composition in some common cellulosic ethanol feedstocks are shown in Table 10.1. Rice and barley straw are low lignin feedstocks, whereas woody materials like pine and eucalyptus are known for higher lignin contents.
Current pilot plants producing ethanol from lignocellulosic material use the residual lignin for energy generation, sequester it as "biochar" or as a carbon sink, or dispose of it as waste. Heat energy from burning lignin can be used to distill ethanol in the plant or can be used as boiler fuel in steam turbine power plants generating electricity. Whereas, in the complete biorefinery concept, lignin is extracted from the residue and supposed to be used in the production of high-value-added chemicals, which would yield higher profits for the bioethanol industry. Nevertheless, potential applications of lignins are closely related to its purity, molecular size, distribution, and the amounts of different chemical functional groups.
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Table 10.1 Lignin contents in common cellulosic ethanol feedstocks.
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Impurities such as carbohydrates and ash will act as obstacles in further applications of lignins as well as depolymerization of lignin to smaller well-defined precursors, which are needed in many refined applications.
There are a number of physicochemical factors which suggest a variety of potential applications for lignin-based products [13,14]:
1. Compatibility with a wide range of industrial chemicals.
2. Presence of aromatic rings providing stability, good mechanical properties, and the possibility of a broad range of chemical transformations.
3. Presence of reactive functional groups such as alcohol and phenol groups allowing facile preparation of graft copolymers.
4. Good rheological and viscoelastic properties for structural materials.
5. Good film-forming ability.
6. Can be obtained in small particle sizes.
7. Hydrophilic or hydrophobic character depending on origin, allowing a wide range of blends to be produced.
Lignin is the main byproduct in a cellulolysis-type cellulosic ethanol plant, therefore applications and proper utilization of lignin is a key factor in the success of the cellulosic ethanol industry, and it is an integral part of the total biorefinery concept. The focus in this chapter is the possible value-added chemicals and polymers that can be derived from lignin, which is likely to be generated in very large quantities in the future cellulosic ethanol plants [14].